The invention concerns an LC structure useful as an RF interrogatable transponder means in electronic security systems for alarming the presence of said LC structure in an RF electromagnetic field within a security zone.
LC structures of the aforementioned type have become known from U.S. Pat. No. 3,913,219, issued Oct. 21, 1975 ("Planar Circuit Fabrication Process", Inventor LICHTBLAU). Said Patent describes a planar circuit comprising a flexible dielectric sheet having an inductively reacting conductive spiral path bonded to one side thereof. Said spiral path terminates, at either ends, into first and second conductive areas which are mutually aligned and spaced by said dielectric sheet, thus forming a lumped capacitor. Said capacitor and said conductive spiral cooperate to form an LC resonant structure. Similar LC structures are described in U.S. Pat. Nos. 3,863,244, 3,967,161, and 4,021,705. All of these LC structures exhibit four essential characteristics. (1) The inductor configures as a single conductive spiral. (2) First conductive material is arranged to react inductively only. (3) Second conductive material is arranged to react capacitively only. (4) An electrical through-contact must be provided through said flexible dielectric sheet. Attempts to make these LC structures small in size detrimentally affect their detectability within security systems, from the following reasons. Wasting on the LC structure's area first metal for reacting inductively only and second metal for reacting capacitively only leads to a highly progressive metal coverage of the LC structure's real estate, the more structure size is reduced, provided said dielectric sheet cannot undergo a certain thickness. The thus high metal coverage of the LC structure's real estate disperforms manifoldly: Firstly, all portions of the conductive spiral path themselves form infinitesimal short circuit loops, resulting in an inherent decrease of inductor's inductance considerably below the maximum inductance attainable with inductor paths configured like the spring of a watch. Consequently, such planar spiral inductor's inductance and Q are detrimentally reduced versus reduction of planar spiral size. Secondly, exactly that metal provided to desirably form a capacitor only, and that are the capacitor plates, in the manner of a much wider short circuit loop effects a deleterious reduction of inductance, too. Hence, larger capacitor plates are required for a resonance at a predetermined frequency, which larger conductive plates even more reduce the structure inductance, and progressively so on. It is understood, that the efficiency of the metal input for attaining a desired resonance and detectability performance output is getting worse versus a decrease of LC structure's size, in a highly progressive manner. Thirdly, the more metal is arranged on a planar LC structure's real estate, the more eddy current counter induction takes place, resulting in an undesired distortion effect on an interrogating RF magnetic field to thus have it bypass the LC structure, rather than guiding an utmost number of field lines through the interior thereof to thereby maximize the effect on an interrogating security system.
U.S. Pat. No. 4,369,557, issued Jan. 25, 1983 ("Process for Fabricating Resonant Tag Circuit Constructions", Inventor VANDEBULT) describes an LC structure which overcomes the need for an electrical contact through an insulative sheet. This LC structure also comprises an inductor and a lumped capacitor. The inductor is formed of two conductive spiral paths bonded to one face of a continuous and flexible dielectric sheet, one spiral path juxtaposed and in electrical communication to the other. The non-juxtaposing ends of said spiral paths terminate in first and second capacitor plates on same face of said dielectric sheet. Said sheet is folded towards itself so that first and second capacitor plates superpose each other in alignment, spaced by the double thickness of said dielectric sheet, the latter thereby also insulating one from another first and second turns of said conductive spiral paths to thus form a continuous dual layer coil of uniform turn direction. Respective inductor portions, i.e. respective turns of first and second spiral paths are substantially misaligned with respect to one another, whereby capacitive coupling between said first and second inductor portions through the double layered dielectric sheet is minimized. The capacitor plates and the double layer of said dielectric sheet are laminated together to thereby form a defined lumped resonance capacitance. A minimized capacitance between inductor turns is attained by that the inductor turn region is not laminated. This is, since turn-to-turn leakage capacitances to result are subject to deviations in response to the degree of mutual misalignment of inductor turns. Thus, the higher the lumped capacitance between lumped capacitor plates is made, the closer tolerances of a resonant frequency can be attained, and vice versa. Hence, in view of the abovementioned distortion effect on a scanning RF electromagnetic field this building scheme comes even shorter since a practically folding dielectric sheet cannot undergo the thickness as known from the Lichtblau construction, however, becomes effective with its double thickness between capacitor plates, which leads to an even higher progressive metallization of a small LC structure's real estate and thus to a lowered inductance and considerable limitations and restrictions of such LC structure in its practical use.
It is understood that when such constructions are required in large quantities, the production costs are of essential significance.